Update docs to reflect the fact that Iterable etc. are now protocols

[JukkaL]

[JukkaL]

@ilevkivskyi I'm working on this.

[ilevkivskyi]

OK, great! Note that I initially had some docs about this in the protocols PR, the section "Predefined protocols in typing module" was removed in 713db0c you can take some text from there.

Sorry, I had not have time recently to work on these issues.